The present invention relates to a wind turbine with frequency ride through capability and a method for operating such a wind turbine.
Wind power plants may be subject to fluctuations of the availability of primary energy due to wind gusts. For this reason, variable speed generators may be used for generating electric power by means of wind power plants because, when using such generators, the energy of wind gusts may not be immediately supplied to the grid but can be stored in the centrifugal masses of the wind power plant by variation of rotational speed. In this manner, the mechanical loads on the wind power plant may be substantially reduced compared to plants with fixed rotational speed, and the mechanical parts may possibly be designed and manufactured lightweight and with reduced costs. Induction generators are sometimes used as variable speed generators, wherein their stator coils are directly coupled to the voltage grid and their rotor windings are driven by the rotor of the wind power plant and are supplied with rotor currents by means of suitable converters. Therein, the frequencies of the supplied rotor currents are controlled in a manner that the sum of the rotor rotational frequency and the rotor current frequency is permanently equal to the grid frequency. For feeding the rotor windings, direct converters coupled to the grid as well as intermediate voltage circuit converters with a grid-sided converter and a rotor power converter coupled thereto via an inductive and/or capacitive reactance can be used.
However, the converters used in the rotor circuit may have certain limitations as to maximum rotor current and/or maximum rotor voltage. For example, a converter sometimes used in the rotor circuit of a wind turbine is an IGBT module in the 1700V voltage class. Such an IGBT module allows a maximum AC rotor voltage of 750 V to 770 V. Furthermore, the IGBT's limitation on the maximum rotor current may require a minimum slip and a minimum rotor voltage. If these limitations are not obeyed, the rotor currents may become too high for the IGBT converters in the rotor circuit and may, therefore, damage the converters.
Wind turbines may be designed for operation under “normal” conditions, i.e. for operation under nominal grid voltage and nominal grid frequency, and may have to fulfill simultaneously several boundary conditions. For example, the rotor voltage Ur depends on the grid voltage Ug and on the generator slip s. The slip s, in turn, depends on the grid frequency fg and on the generator rotor speed nmech. Particularly, the slip s is proportional to the grid frequency fg in the overexcited range and reciprocally proportional in the under excited range. Especially, the maximum dynamic speed range, the nominal speed point nnom and the maximum allowable rotor voltage Ur may be designed under the assumption of “normal” grid conditions, i.e. for nominal grid frequency (fg=50 Hz in Europe, fg=60 Hz in USA and parts of Japan) and nominal grid voltage Ug. Therefore, the simultaneous occurrence of several abnormal operating conditions can lead to a shut-down of the wind turbine, especially during gusty wind conditions, due to exceedingly high rotor currents.
One known ride-through strategy for such abnormal operating conditions where exceedingly high or low grid frequencies may occur is to adjust the power factor to a more inductive range. However, this strategy can possibly only be applied within a relatively narrow range of frequency transients. Particularly, adjusting the power factor to the more inductive range may lower the grid voltage Ug. Therefore, this strategy may be undesirable in cases of under voltage where further lowering of the voltage may not be allowed.